IGNEOUS ROCKS Where do igneous rocks form? Understanding Earth 6 th - - PowerPoint PPT Presentation

IGNEOUS ROCKS

FUNDAMENTALS OF EARTH SCIENCE I FALL SEMESTER 2018

 Where do igneous rocks form?

Understanding Earth 6th Ed.

Slow cooling Rapid cooling Cooling of magma/lava within the lithosphere near or on Earth’s surface Crystallizationof minerals Formation of an igneous rock INTRUSIVE IGNEOUS ROCKS EXTRUSIVE IGNEOUS ROCKS Coarser-grained texture Finer-grained texture e.g. granite e.g. basalt

 Classification of igneous rocks

• 1. TEXTURE

Understanding Earth 6th Ed.

Different types of igneous rocks identified based on the texture

Understanding Earth 6th Ed.

FELSIC MAFIC

Igneous rocks enriched in SiO2 and silicates rich in Al, K, Na Quartz (SiO2) Orthoclase (K-rich feldspar) Plagioclase (Na/Ca-rich feldspar) Muscovite (K-rich mica) Example: granite (continental crust) Light color Igneous rocks enriched in silicates rich in Fe, Mg Biotite (mica) Amphibole group Pyroxene group Olivine Example: basalt (oceanic crust) Dark color CONTINUUM Felsic : feldspar-silica Mafic : magnesium-ferric

• 2. CHEMICAL AND MINERALOGICAL COMPOSITION
• Felsicvs. mafic compositions

Oceanic crust

Mantle

0-7 km 35-60 km Fe, Mg > Fe, Mg >> Al, K, Na > Olivine: (Mg, Fe)2SiO4 Pyroxene: XY(Si,Al)2O6 Enstatite (MgSiO3) and ferrosilite (FeSiO3)

Basalt Granite

3.0 g/cm3 2.8 g/cm3 3.4 g/cm3

Continental crust

(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6 Augite

Peridotite

CaAl2Si2O8 Anorthite: Ca-rich plagioclase Pyroxene Quartz: SiO2 KAlSi3O8 Orthoclase:

Intrusive

DARK color

Crystallize and melt at different T !!!

ABUNDANT IN THE CONTINENTAL CRUST DOMINANT IN EARTH’S UPPER MANTLE

Extrusive

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ABUNDANT IN THE OCEANIC CRUST

(underlying ocean floor)

LIGHT color

http://www.gso.uri.edu/lava/MagmaProperties/properties.html

(Mg, Fe)SiO3 (Mg, Fe)2SiO4 NaAlSi3O8 CaAl2Si2O8 Albite Anorthite SiO2 KAlSi3O8

Melting of mantle rocks Production of basaltic magmas at

• ceanic hot spots

Production of more felsic magmas at

• cean-continent

subduction zones Low-viscosity magmas High-viscosity magmas (+ rich in volatiles) Felsic continental crust Mafic oceanic crust LOW RISK OF EXPLOSION HIGH RISK OF EXPLOSION H2O-rich CO2-rich

CaCO3 + SiO2 → CaSiO3 + CO2

Understanding Earth 6th Ed. www.britannica.com

“PAHOEHOE” LAVA “AA” LAVA

www.britannica.com

“PILLOW” LAVA

Basaltic volcanism (mafic composition)

Shield volcanoes Hawaii hot spot

Understanding Earth 6th Ed.

MOUNT CHAITÉN (Chile) RHYOLITIC LAVA DOME (Oregon, USA)

Understanding Earth 6th Ed. Sam Beebe (Wikipedia) USGS

Rhyolitic volcanism (felsic composition)

Volcanic domes

 Processes of magma formation

Factors controlling magma production:

• 1. Temperature
• All minerals do not melt at the same temperature (felsic vs. mafic)
• 2. Pressure
• Lower pressures result in lower melting temperatures

→ Lowering the pressure facilitates melting!

• 3. Water content
• Increased water content results in lower melting temperatures

→ Adding water facilitates melting!

Compositionof partial melt 1 (magma 1) = composition of partial melt 2 (magma 2)

• 1. Role of temperature

Different minerals melt at different temperatures

MAGMA 1 MAGMA 2

T1 T2

Parent rock of identical composition % melting 1 < % melting 2 Temperature 1 (T1) < Temperature 2 (T2)

• 2. Role of pressure
• High pressures deep in the Earth’s interior prevent rocks from melting
• Hot mantle rock begins to melt when it rises beneath mid-ocean ridges and

hotspots when the pressure drops = Decompression melting

HOT SPOT Mid Ocean Ridge (MOR)

Ascending hot mantle rock Ascending hot mantle rock

Decompression melting Decompression melting

Sedimentary rocks carried by the subducting plate have a high water content in the open space between grains (pores) and in clay minerals.

• 3. Role of water

Water-induced melting

Subduction

Lithosphere Lithosphere

High water content triggers melting at relatively low temperature "Cold" subducting

• ceanic lithosphere

Water molecules disrupt chemical bonds and lower the melting temperature of silicate minerals.

Na+

,Ca2+, H2O

Hydrous Aluminium Phylosilicates (smectite group) e.g. montmorillonite → (Na, Ca)0.33(Al, Mg)2Si4O10(HO)2.nH2O

Mg2+,

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Rock temperature vs. depth (T increases with depth)

http://www.ei.lehigh.edu/learners/tectonics/heatflow/heatflow2.html

WATER-INDUCED MELTING DECOMPRESSION MELTING

Oceanic lithosphere Oceanic crust Mantle Asthenosphere (main original source of magma)

How magma forms: Geothermal gradient & rock solidus

Red line (geotherm): Temperature at which the rock starts to melt Green line (solidus):

sol liq

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Magma is less dense than surrounding solid rocks and rises through fissures in the rock or by melting its way up. Magma accumulates in large magma chambers in the crust.

1. Magma chamber (pluton) 2. Surrounding rocks melt and influence magma composition 3. Different minerals crystallize at different temperatures which influence the composition

• f the remaining melt

4. Sill (horizontal sheet-like intrusion) 5. Dyke (vertical sheet-like intrusion) 6. Central vent 7. Side dent 8. Lava flow 9. Pyroclasts

Forms of igneous intrusions

 Magma crystallization and formation of igneous rocks

• 1. Crystal fractionation
• Minerals crystallize at different temperatures. Minerals that crystallize first

in the magma chamber tend to settle down first. This is called crystal

• fractionation. This process results in the formation of igneous rocks of

different compositions. One single parent magma can therefore produce different igneous rocks. Change in magma composition during crystallization is called magmatic differentiation.

• 2. Crustal contamination
• Changes in magma composition as the magma travels in the crust and

incorporate pieces of the surrounding crustal rocks.

• 3. Magma mixing
• The mixing of magmas with different chemical compositions may lead to

the formation of igneous rocks whose compositions differ from the rocks that would have been produced if the two magmas had crystallized separately without mixing.

How can we explain the diversity of igneous rocks?

The Bowen’s reaction series (established experimentally)

Understanding Earth 6th Ed.

• 1. Crystal fractionation

Olivine crystallized first and settled down at bottom 1 2 3 Order that follows Bowen’s reaction series

Minerals crystallizing first tend to settle down first in magmatic intrusions which means that layers of igneous rocks of different compositions can form. Crystal settling rate also depends on density and size of crystals and the viscosity of the remaining magma (+ turbulences in magma chamber)

Understanding Earth 6th Ed.

Magma derived from mantle rocks Contamination by sedimentary rocks and continental crustal rocks

Original magma derived mostly from mantle rocks ULTRAMAFIC

• 2. Crustal contamination

Mafic Mafic to felsic Mafic to intermediate

Contamination by

• ceanic crustal rocks

(including sediments)

• 3. Magma mixing

Understanding Earth 6th Ed.

NB: Some magmas are immiscible, which means they cannot mix (like oil and water)

Craig et al. (2011)

Igneous differentiation can lead to the segregation of valuable minerals in layered intrusions.

Layers of chromite (black), Bushveld (photo: Jackie

Gauntlett, blogs.agu.org)

Iron-titanium oxide (ilmenite) mining in Norway (wikipedia)

Intrusive equivalent of basalt

Solid mantle rock!!!

 Formation of oceanic crust at mid-ocean ridges

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Understanding Earth 6th Ed.

www.omantourism.gov.om

Large ophiolite outcrops

• ccur in Oman

Modified from Schreurs and Milson (2006)

Subduction Obduction 100-80 x 106 yr ago Oman

• Cont. litho.

Oceanic litho.

Copper sulfide deposits related to black smokers